Capacitive incremental position sensor

A capacitive incremental position sensor is a family of Capteur S using the capacitive effect to detect a variation of short distances. It is very generally carried out with a electrode, in the shape of disc, planes surrounded by a guard ring isolated from the central electrode. The electrode forms with the part to measure conducting a plane condenser. One also finds it in the shape of two imbricated combs, which increases capacitive surface when a not very thick sensor is necessary.

Theory

The determination of the distance D is carried out by measuring the capacity C of the condenser, they are bound by the equation:

$C= \ frac {\ epsilon S} {D}$

With

ε is the Permittivité dielectric what exists between the sensor and the part to be measured

S is the surface of the electrode of the sensor

D is the distance to be measured

The measurement of the capacity of the condenser can be realized by injecting an alternative course to him and by measuring the tension which appears on its terminals. They are bound by the relation:

$V= \ frac {I} {I \ Omega C} = \ frac {I.d} {I. \ Omega. \ epsilon. S}$

This tension is proportional to the distance D, ω = 2πf is the pulsation of the current of food.

The electrode of guard which is placed around that of measurement has its potential which is carried to the same value in order to improve the linearity while making the lines of field normal to the central electrode, it eliminates the effects edge.

Applications

The capacitive sensor is very much used for weak displacements, like for the miniaturized sensors (MEMS), such as the Accéléromètre S.

See too

Sensor of proximity

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